Process for chemical-mechanical polishing of metal substrates

ABSTRACT

Abrasive composition for the chemical-mechanical polishing in one stage of substrates used in the microelectronics semiconductors industry containing at least one metal layer and one insulator layer, comprising an acid aqueous suspension of individualized particles of colloidal silica, not linked to each other by siloxane bonds, having a mean particle diameter of between 5 and 20 nm and an oxidizing agent, and chemical-mechanical polishing process using such a composition.

The present invention relates to a chemical-mechanical polishing processand to an abrasive composition used in the polishing and flattening ofsurfaces in the micro-electronics semiconductors industry, and moreparticularly of substrates comprising at least one metal layer and oneinsulator layer.

Integrated circuits are constituted by a large number of active devices(transistors for example).

These active devices, isolated from each other, must be interconnectedby means of interconnections and vias at multiple levels.

Different metals are used to produce these interconnections and it isoften necessary to polish these metal layers during the manufacturingprocess.

Chemical-mechanical polishing (CMP) is a technology greatly used in themicro-electronics semiconductors industry for polishing different layersof materials found on or in semiconductor substrates.

The production of interconnections or of vias is carried out as follows:

-   -   an insulator layer is deposited on a support,    -   trenches or holes are produced in the insulator,    -   in general, a barrier layer is then deposited on the free upper        surface of the insulator, which thus covers the insulator and        the bottom and sides of the trenches,    -   the same procedure is then carried out with a metal layer,    -   an abrasion is then carried out in order to eliminate from the        upper surface of the insulator the metal and, if necessary, the        product forming a barrier.

A flat surface is obtained because the trenches have been filled.

These operations can be repeated in order to obtain several successivestrata of metal and of insulator (see FIGS. 3 and 4).

For the chemical-mechanical polishing of substrates containing at leastone metal layer and one insulator layer, two phenomena must be avoided:

-   -   an attack of the subjacent insulator layer, also called erosion.        This locally introduces ridges and acts contrary to the sought        objective of flattening.    -   an over-polishing of the metal layer, also called “dishing”        which will also generate ridges.

It is therefore desirable to find abrasive compositions making itpossible to obtain a high polishing speed of the metal layer, anexcellent surface state of the metal layer and of the insulator layer, auniformity of polishing of the metal layer and good selectivity betweenthe polishing of the metal and that of the insulator.

Furthermore, the composition must be as simple as possible, that is tosay it must contain a minimum number of constituents.

The abrasive compositions known at present for polishing substratescontaining at least one metal layer and one insulator layer essentiallycontain an abrasive, an oxidizing agent and one or more additives makingit possible to modify the selectivity of polishing the layers (seeAdvanced Silicon Processing—2002, Chapter 5, page 57, Lattice Press).

U.S. Pat. No. 5,916,855, U.S. Pat. No. 6,117,783 and U.S. Pat. No.5,244,534 describe the use of polishing slurries containing aluminiumoxide particles. These polishing compositions have good polishing speedsof the metal layer as well as a good selectivity with respect to theinsulator layer. However, these aluminium oxide particles are not verystable over time and have a tendency to form agglomerates causingmicro-scratches on the surface of the polished layers.

It is therefore necessary to have recourse to a second finishingpolishing stage in order to eliminate these scratches (see AdvancedSilicon Processing-2002, Chapter 5, Page 58, Lattice Press).

Colloidal particles of pyrogenated silica (or fumed silica) are alsoused as in EP-A-708160, EP-A-844290 or EP-A-896042.

The use of these particles has a certain number of drawbacks. Firstly,because of the wide distribution of the length of the aggregates, thereis a tendency toward the sedimentation of the latter over the course oftime and therefore a lack of stability. Furthermore, this low stabilitycan be minimized only by a constant agitation of the suspension, whichdoes not facilitate the utilization.

More recently, the use of colloidal silica for the polishing of metalsis described for example in the patent WO 00/30154. However, a poormetal/insulator polishing selectivity is observed.

Now, the applicant has observed surprisingly and unexpectedly that theuse of small-sized individualized particles of colloidal silica, notlinked to each other by siloxane bonds, in acid aqueous suspension, andin the presence of an oxidizing agent, even in the absence of polishingadditives such as an anti-corrosion agent or a cleaning agent, made itpossible to obtain directly a high polishing speed of the metal layer,an excellent surface state of the metal layer and of the insulatorlayer, a uniformity of polishing of the metal layer and good selectivitybetween the polishing of the metal and of the insulator without havingto carry out a second supplementary stage of finishing polishing(referred to in English as “post metal polishing” or buff polishing”).

That is why the present application relates to a chemical-mechanicalpolishing process (also called CMP) for substrates used in themicroelectronics semiconductors industry and containing at least onemetal layer and one insulator layer, separated, if necessary, by abarrier layer, in which the metal layer or layers and the barrier layeror layers are subjected to friction using a polishing pad by moving thesubstrate with respect to the pad and by pressing the substrate againstthe said pad, and an abrasive composition is deposited on the pad duringthe polishing, characterized in that the said process is carried out ina single stage, in that the abrasive composition comprises:

-   -   an acid aqueous suspension of individualized particles of        colloidal silica, not linked to each other by siloxane bonds,        having a mean particle diameter of between 5 and 20 nm,    -   an oxidizing agent,        and in that the metal layer and, if applicable, the barrier        layer, is or are eliminated from the surface of the insulator in        order to obtain a metal and insulator surface not requiring any        finishing polishing.

With regard to substrates, it is possible to mention in particular,according to the present invention, substrates in which the metal layeris produced from a metal selected from the group comprising aluminium,copper and tungsten, and preferably the latter, and the barrier layer isproduced from a material selected from the group including titanium,tantalum, titanium nitride, tantalum nitride and any combination oralloy of at least two of them.

The insulation layer is in particular selected from the group comprisingsilicon oxide, tetraethoxysilane oxide, phosphosilicate glass,borophosphosilicate glass and polymers with a low dielectric constant,and more particularly from the group comprising silicon oxide,tetraethoxysilane oxide, phosphosilicate glass and borophosphosilicateglass.

The abrasive composition can be poured on the polishing pad continuouslyor sequentially, regularly or irregularly. The composition is thusdragged to the interface between the polishing pad and the substrate andcan thus polish the surface of the latter.

The polishing pad is normally made from organic polymer of thepolyurethane type.

During its use, the above acid aqueous suspension of colloidal silica isused with a concentration by weight of silica of between 0.1 and 15%, inparticular of between 1 and 10%, and especially between 2 and 5%.

The acid aqueous suspension of individualized particles of colloidalsilica, not linked to each other by siloxane bonds, is preferably usedat a pH of less than 5, in particular of between 1 and 5, and especiallyof between 1.5 and 3.

The acidification can be obtained in particular by the addition of amineral acid such as nitric acid or phosphoric acid.

The mean diameter of the individualized particles of colloidal silica,not linked to each other by siloxane bonds, is advantageously between 7and 15 nm and especially between 9 and 12 nm.

The particles of colloidal silica which can be used in the presentinvention can be obtained by using wet processes starting with rawmaterials such as tetramethyl- or tetraethyl-orthosilicate, or evensodium or potassium silicate.

These processes known to those skilled in the art are described in “K.K. Iler, The Chemistry of Silica, Chapter 9, Pages 331 to 343, Publishedby Wiley Interscience, 1979”.

Aqueous suspensions of individualized particles of colloidal silica, notlinked to each other by siloxane bonds and stable in time, are directlyobtained by these processes.

It is necessary here to recall the fundamental differences existingbetween individualized particles of colloidal silica, not linked to eachother by siloxane bonds, and other types of silica such as pyrogenatedsilicas, also called fumed silica.

These particles of pyrogenated silica can be obtained by using dryprocesses. They are prepared for example by combustion oftetrachlorosilane of high purity with hydrogen and oxygen in acombustion chamber at high temperature. The particles are notindividualized but exist in the form of aggregates or agglomerates ofprimary particles of spherical silica of 5 to 50 nm which formaggregates of particles of length generally within the range 50 to 500nm. The particles obtained in powder form must be dispersed in thepolishing medium (water for example).

The structure of the different silicas is illustrated hereafter in theFIGS. 1 and 2.

Furthermore, hydrogen peroxide is mentioned regularly as an oxidizingagent for the metals in the polishing slurries.

However, there are several drawbacks in the use of hydrogen peroxide. Itdecomposes over time, which leads to a reduction in the activity of theabrasive composition. Consequently, for transport and storage, aspecific package for the abrasive composition and a specific package forthe hydrogen peroxide are required and the hydrogen peroxide is added tothe abrasive composition at the last moment before use.

Under preferred conditions of implementing the process described above,the oxidizing agent is an iodate, in particular potassium or sodiumiodate.

The oxidizing agent is for example used at a concentration by weight ofbetween 0.1 and 15% in the composition ready for use, in particular ofbetween 0.1 and 6%, and advantageously of between 2 and 5%.

The abrasive compositions to which the present invention relates havevery useful properties.

They make it possible to obtain a high polishing speed of the metallayer.

They also make it possible to obtain an excellent surface state of themetal layer and of the insulator layer.

The polishing of the metal layer is uniform.

It should also be noted that there is a good selectivity between thepolishing of the metal and that of the insulator.

They can be ready for use, depending on the oxidizing agent used.

That is why the present application also relates to an abrasivecomposition for chemical-mechanical polishing, characterized in that thesaid abrasive composition comprises or preferably consists of:

-   -   an acid aqueous suspension of individualized particles of        colloidal silica, not linked to each other by siloxane bonds,        having a mean particle diameter of between 5 and 20 nm,    -   an oxidizing agent.

The present application also relates to the above ready-for-use abrasivecomposition, not requiring the addition of the oxidizing agent at themoment of use, characterized in that the oxidizing agent is an iodate,in particular potassium iodate or sodium iodate.

The abrasive compositions of the present invention can optionallycontain additives such as surface-active agents, complexing agents,corrosion inhibitors, additives that modify the polishing selectivity,buffering agents, stabilizers, bactericides, fungicides and biocides.

However, under other preferred conditions of the invention, the abrasivecomposition used comprises at most two polishing additives, such asanti-corrosion agents or cleaning agents, preferably one additive atmost, and in particular substantially comprising no additive, that is tosay less than 0.08% by weight, in particular less than 0.05% by weightand especially no additive at all.

In particular, an abrasive composition according to the inventioncontains:

-   -   substantially no anti-corrosion agent,    -   substantially no cleaning agent,    -   substantially no anti-corrosion agent and no cleaning agent.

The preferred conditions of implementing the processes described abovealso apply to the other objects to which the invention relates, asmentioned above, in particular to abrasive compositions.

The structure of colloidal silica whose particles are individualized andnot linked to each other by siloxane bonds is illustrated in FIG. 1 by aphotograph taken with an electronic microscope.

The structure of fumed silica is illustrated in FIG. 2 by a photographtaken with an electronic microscope.

FIG. 3 shows a stratum of layers deposited on a support.

FIG. 4 shows the stratum of FIG. 3 after chemical-mechanical polishingaccording to the invention.

The support layer is not shown. Layer 1 is the insulator layer, layer 2is a barrier and layer 3 is a metal.

The following examples illustrate the present application. “%” alwaysmeans “% by weight”.

EXAMPLE 1

To 8,370 g of an acid aqueous suspension of colloidal silica having aconcentration by weight of silica of 20%, a mean particle diameter of 12nm and a pH of 2.4 (marketed under the name KIebosol® PL 152H-12), isadded 53,630 g of an 4.5% aqueous solution of KlO₃. 62,000 g of an acidpolishing composition are obtained containing 2.7% of colloidal silicahaving a mean particle diameter of 12 nm and 3.9% of KlO₃ as anoxidizing agent. The pH of the abrasive composition is adjusted to 2.1by adding 65% nitric acid.

EXAMPLE 2

An acid polishing composition is prepared containing 2.7% of colloidalsilica having a mean particle diameter of 7 nm, a pH of 2.1 and 3.9% ofKlO₃ as an oxidizing agent by proceeding as in example 1 but using anacid aqueous suspension of colloidal silica having a mean particlediameter of 7 nm.

EXAMPLE 3

An acid polishing composition is prepared containing 2.7% of colloidalsilica having a mean particle diameter of 9 nm, a pH of 2.1 and 3.9% ofKlO₃ as an oxidizing agent by proceeding as in example 1 but using anacid aqueous suspension of colloidal silica having a mean particlediameter of 9 nm.

EXAMPLE 4

To 8,370 g of an acid aqueous suspension of colloidal silica having aconcentration of silica of 20%, a mean particle diameter of 12 nm and apH of 2.4 (marketed under the name Klébosol® PL 152H-12), is added16,533 g of a 30% aqueous solution of H₂O₂ and then a sufficientquantity of water to reach 62,000 g is added. 62,000 g of an acidpolishing composition are obtained containing 2.7% of colloidal silicahaving a mean particle diameter of 12 nm and 8% of H₂O₂ as an oxidizingagent. The pH of the abrasive composition is adjusted to 2.1 by adding65% nitric acid.

COMPARATIVE EXAMPLE C1

To 5,580 g of an acid aqueous suspension of colloidal silica having aconcentration of silica of 30%, a mean particle diameter of 25 nm(marketed under the name Klébosol) PL 150H-25), are added 56,420 g of a4.3% aqueous solution of KlO₃ 3%. 62,000 g of an acid polishingcomposition containing 2.7% of colloidal silica having a mean particlediameter of 25 nm and 3.9% of KlO₃ as an oxidizing agent are obtained.The pH of the abrasive composition is adjusted to 2.1 by adding 65%nitric acid.

COMPARATIVE EXAMPLE C2

An acid polishing composition is prepared containing 2.7% of colloidalsilica having a mean particle diameter of 50 nm, a pH of 2.1 and 3.9% ofKlO₃ as an oxidizing agent by proceeding as in comparative example C1but using an acid aqueous suspension of colloidal silica having a meanparticle diameter of 50 nm (marketed under the name Klébosol®) PL150H-50).

COMPARATIVE EXAMPLE C3

To 46,500 g of an acid aqueous suspension of colloidal silica having aconcentration of silica of 20%, a mean particle diameter of 12 nm and apH of 2.4 (marketed under the name Klébosol®) PL 152H-12), are added13,082 g of water and then 2,418 g of KlO₃ are dissolved whilststirring. 62,000 g of an acid polishing composition containing 15% ofcolloidal silica having a mean particle diameter of 12 nm and 3.9% ofKlO₃ as an oxidizing agent are obtained. The pH of the abrasivecomposition is adjusted to 2.1 by adding 65% nitric acid.

Examples of Application

For polishing the insulator, silicon substrates are used upon which isdeposited by plasma a layer of tetraethoxysilane oxide (TEOS) of about3200 Å. For polishing the tungsten, silicon substrates are used uponwhich is deposited a layer of TEOS (about 6000 Å) and then a layer oftitanium (about 300 Å), then a layer of titanium nitride (about 1000 Å)and finally a layer of tungsten (about 8000 Å). The layer of TEOS and oftungsten is then polished on a Mecapol® E460 polishing machine under thefollowing conditions: Down force: 0.48 bar Plate speed:   70 rpm Headspeed:   50 rpm Abrasive flow rate:  150 ml/min Polishing pad: IC 1000 Kwith Rodel furrows.

The test of the speed of attack of the TEOS and tungsten layer is thencarried out. The speed of attack is measured by the difference inthickness before and after polishing per minute of polishing. It isexpressed in A/min. In order to measure the thickness of the TEOS, aNANOSPEC AFT-6100 spectro-reflectometer is used with measurement at 49points. In order to measure the thickness of the tungsten, an RS75KLA-Tencor OmniMap® 4-probe resistivimeter is used with measurement at49 points.

Experiment 1

Effect of Particles Size

A layer of tungsten and TEOS is polished under the conditions mentionedabove using abrasive compositions comprising an acid aqueous suspensionof individualized colloidal silica particles, not linked to each otherby siloxane bonds. The abrasive compositions contain 2.7% of colloidalsilica, have a pH of 2.1 and contain 3.9% of KlO₃. The results obtainedare shown in Table 1. TABLE 1 Mean diameter Tungsten Tungsten/ of theabrasive polishing speed TEOS polishing TEOS Tests particles (nm)(Å/min) speed (Å/min) Selectivity 1 7 2400 90 26/1  2 9 2800 110 25/1  312 3300 250 13/1  4 25 2000 1000 2/1 5 50 1000 1700 0.6/1  

The different tests in Table 1 show the effect of abrasive particlessize on the polishing speed of the metal layer and of the insulatorlayer and the metal to insulator polishing selectivity. The followingcan be observed, for the small particles sizes (7, 9 and 12 nm):

-   -   a good polishing speed of the tungsten layer    -   a low polishing speed of the TEOS        which reveals a good tungsten/TEOS polishing selectivity. On the        contrary, the following can be observed for the larger particles        sizes:    -   a significant reduction in the tungsten polishing speed    -   an increased TEOS polishing speed        which reveals a very poor tungsten/TEOS polishing selectivity.        Experiment 2        Effect of the pH Value

Under the same conditions as before, a layer of TEOS and tungsten waspolished using abrasive compositions comprising an aqueous suspension ofindividualized colloidal silica particles, not linked to each other bysiloxane bonds. The abrasive compositions contain 2.7% of colloidalsilica, with a mean particle diameter of 9 nm and contain 3.9% of KlO₃.The results obtained are shown in Table 2. TABLE 2 Tungsten polishingTEOS polishing Tungsten/TEOS Tests pH speed (Å/min) speed (Å/min)Selectivity 6 7 800 460 1.7/1   7 2.1 2800 110 25/1 

Examining Table 2 reveals that it is necessary to have an acid polishingcomposition in order to obtain a good tungsten polishing speed as wellas a good selectivity.

EXAMPLE 3

Effect of Particles Concentration

A layer of TEOS and tungsten was polished under the same conditions asbefore using abrasive compositions comprising an acid aqueous suspensionof individualized colloidal silica particles, not linked to each otherby siloxane bonds. The abrasive compositions, of pH 2.1, containcolloidal silica particles with a mean particle diameter of 12 nm andcontain 3.9% of KlO₃. The results obtained are shown in Table 3. TABLE 3Concentration Tungsten Tungsten/ by weight of polishing speed TEOSpolishing TEOS Tests colloidal silica (Å/min) speed (Å/min) Selectivity8  15% 3500 2000 1.7/1   9 2.7% 3300  250 13/1 

The results in Table 3 indicate that it is preferable to use a diluteacid aqueous suspension of colloidal silica in order to obtain a hightungsten polishing speed and good tungsten/TEOS selectivity.

1. A chemical-mechanical polishing process for substrates used in themicro-electronics semiconductors industry comprising at least one metallayer and one insulator layer, separated, if necessary, by a barrierlayer, in which the metal layer or layers and the barrier layer orlayers are subjected to friction using a polishing pad by moving thesubstrate with respect to the pad and by pressing the substrate againstsaid pad, and an abrasive composition is deposited on the pad during thepolishing, wherein said process is carried out in a single stage, saidabrasive composition comprising: an acid aqueous suspension ofindividualized particles of colloidal silica, not linked to each otherby siloxane bonds, having a mean particle diameter of between 5 and 20nm, and having a concentration by weight of silica of between 1 and 10%,and an oxidizing agent, and in that the metal layer and, if applicable,the barrier layer, is or are eliminated from the surface of theinsulator in order to obtain a metal and insulator surface not requiringany finishing polishing.
 2. A chemical-mechanical polishing processaccording to claim 1, wherein the metal layer is produced from a metalselected from the group consisting of aluminum, copper and tungsten, andthat the barrier layer is produced from a material selected from thegroup consisting of titanium, tantalum, titanium nitride, tantalumnitride and any combination or alloy of at least two of them.
 3. Achemical-mechanical polishing process according to claim 1 wherein theinsulator layer is selected from the group consisting Of silicon oxide,tetraethoxysilane oxide, phosphosilicate glass, borophosphosilicateglass and polymers with a low dielectric constant.
 4. Achemical-mechanical polishing process according to claim 1 wherein theoxidizing agent is an iodate.
 5. A chemical-mechanical polishing processaccording to claim 1 wherein the mean diameter of the individualizedparticles of colloidal silica, not linked to each other by siloxanebonds, is between 7 and 15 nm.
 6. A chemical-mechanical polishingprocess according to claim 1, wherein the acid aqueous suspension ofcolloidal silica is used at a pH of between 1 and
 5. 7. An abrasivecomposition for the chemical-mechanical polishing in one stage ofsubstrates used in the microelectronics semiconductors industrycontaining at least one metal layer and one insulator layer, whereinsaid abrasive composition comprises: an acid aqueous suspension ofindividualized particles of colloidal silica, not linked to each otherby siloxane bonds, having a mean particle diameter of between 5 and 20nm, and having a concentration by weight of silica of between 1 and 10%,and an oxidizing agent, and the abrasive composition is substantiallyfree of anti-corrosion agent (<0.05% by weight).
 8. A compositionaccording to claim 7, wherein the oxidizing agent is an iodate.
 9. Acomposition according to claim 7, wherein the mean diameter of theindividualized particles of colloidal silica, not linked to each otherby siloxane bonds, is between 7 and 15 nm.
 10. A composition accordingto claim 7, wherein the acid aqueous suspension of colloidal silica hasa concentration by weight of silica of between 2 and 5%.
 11. Achemical-mechanical polishing process according to claim 4, wherein theiodate is selected from the group consisting of potassium iodate andsodium iodate.
 12. A chemical-mechanical polishing process according toclaim 1, wherein the oxidizing agent is used at a concentration byweight of between 0.1 and 15%.
 13. A chemical-mechanical polishingprocess according to claim 12, wherein the oxidizing agent is used at aconcentration by weight of between 2 and 5%.
 14. A chemical-mechanicalpolishing process according to claim 5, wherein the mean diameter of theindividualized particles of colloidal silica, not linked to each otherby siloxane bonds, is between 9 and 12 nm.
 15. A chemical-mechanicalpolishing process according to claim 1, wherein the acid aqueoussuspension of colloidal silica is used at a concentration by weight ofsilica of between 2 and 5%.
 16. A chemical-mechanical polishing processaccording to claim 6, wherein the acid aqueous suspension of colloidalsilica is used at a pH of between 1.5 and
 3. 17. A composition accordingto claim 8 wherein the iodate is selected from the group consisting ofpotassium iodate and sodium iodate.
 18. A composition according to claim7 wherein the oxidizing agent is used at a concentration by weight ofbetween 0.1 and 15%.
 19. A composition according to claim 18, whereinthe oxidizing agent is used at a concentration by weight of between 2and 5%.
 20. A composition according to claim 9, wherein the meandiameter of the individualized particles of colloidal silica, not linkedto each other by siloxane bonds, is between 9 and 12 nm.
 21. Acomposition according to claim 7 wherein the acid aqueous suspension ofcolloidal silica has a pH of between 1 and
 5. 22. A compositionaccording to claim 21 wherein the acid aqueous suspension of colloidalsilica has a pH of between 1.5 and 3.